The invention relates to a projection system and to a method of operating a projection system.
A sequence of colors is generated with the aid of the lamp here, and the current strength of an electrical operating current supplied to the lamp is varied in time. Such a variation may serve in particular for stabilizing a lamp arc. In such a projection system, accordingly, means are provided for controlling the current strength of the electrical operating current supplied to the lamp in time, preferably for stabilizing the light arc, and means for generating a sequence of colors by means of the lamp. The lamp used may be in particular a high intensity discharge lamp.
The above projection systems have been known for a considerable time in various forms, for example from WO 95/11572, for example as DLP (digital light processing) projection systems. They usually comprise at least one high intensity discharge lamp (HID lamp) by means of which an array is illuminated, this array being a so-called deformable mirror device or digital mirror device, DMD for short. The individual mirrors of the array form a raster by means of which picture samples are generated and, for example, video images can be displayed. For this purpose, the individual mirrors are pivoted such that they appear bright or dark to the observation, which observation usually does not take place directly. Instead, at least a lens, a deflection mirror, and a picture screen or projection screen are usually present in the radiation path between the array and the eye of an observer.
To generate colored pictures, a fast-rotating color disc is provided in the projection systems, which disc usually comprises at least three segments with the basic colors required for an additive color mixing, i.e. for example red, green, and blue. The colored segments of the color disc usually comprise dichroic filters (so the filter discs are sometimes also referred to as filter wheels), which transmit only light of a given color, so that the light passing through them appears to be, for example, red or green. Colored light rays arriving shortly one after the other are joined together in the eye so as to form a new color impression. It is accordingly possible to generate pictures of any color by means of the fast-rotating color disc and a corresponding control of the individual mirrors.
An alternative for a color wheel is, for example, an electro-optically switchable filter which can be switched in its entirety between various colors and white through the application of a control voltage. Such an element is described, for example, in xe2x80x9cProgress in Field Sequential Color Shuttered Technologyxe2x80x9d by G. Scharp et al. in Proc. SPIE vol. 3013, pp. 107 to 111, 1997.
The rotating color disc or the means for generating a sequential color series may be arranged in such projection systems in principle both downstream and upstream of the DMD in the radiation path from the lamp to the eye. Usually, however, they are provided between the lamp and the DMD.
The known projection systems usually require high intensity discharge lamps with a constant luminous flux, i.e. the lamp must always supply the same amount of light. An example of such a system is found in U.S. Pat. No. 5,680,180. Such HID lamps, however, are problematic as regards the stability of the discharge arc and the operational life of the electrodes. A method of operating a HID lamp with a short light arc is known from U.S. Pat. No. 5,608,294, where the stability of the light arc and the life of the electrodes is substantially improved. The operating current is varied therein such that a stabilization of the lamp arc is achieved. This method, however, cannot be applied to the known projection systems because these systems usually require a constant luminous flux.
It is known from WO 95/11572 to vary the current strength of the operating current of the lamp so as to compensate for color drifts caused by the system and to adjust the color balance. According to the teachings of WO 95/11572, it is essential to adapt the current strength of the operating current of the HID lamp to the requirements of the projection system. This detracts from the lamp properties; in particular, problems arise with the stability of the light arc and electrodes.
In view of the above, the invention has for its object to provide a projection system and a method of operating a projection system which render it possible to operate the HID lamp used with optimized power and operational life without undesirable artefacts arising owing to the non-constant luminous flux, and in particular without problems arising in the color reproduction.
This object is achieved by means of a method and a projection system having the characteristics as defined in the respective main claims. Advantageous implementations and further embodiments are given in the dependent claims.
It should be noted here that the variation of the current strength according to the invention does lead to a certain amount of pulsatory operation of the lamp, but that this does not refer to the type of lamp operation usually denoted xe2x80x9cpulsatory operationxe2x80x9d in which a lamp is operated during very short time periods in quick repetition, but does not supply any light for a major portion of time. Such lamps are also sometimes used in projection systems (cf. for example EP 0 865 210 A2, WO 97/24871, or U.S. Pat. No. 5,428,408). The xe2x80x9cpulsatory operationxe2x80x9d proposed here relates instead to a special form of the current strength gradient in time which leads to a stabilization of the lamp arc. The lamp does provide light here at all times, or at least substantially at all times, but the quantity of light is not constant in time; instead, it follows the current waveform necessary for stabilization.
It is obvious in this connection that the means according to the invention for generating a sequence of colors may be constructed in accordance with any suitable type. It is essential that such a device is capable of generating a sequence of different colors, including white if necessary. In addition, the invention is not limited to a DMD, but it may also comprise, for example, displays or projection systems in which the mirrors are replaced by ferroelectric LCDs. The invention is particularly suitable for projection systems in which colors as well as any desired grey tones are generated through a time-modulated switching of a binary display element within the framework of a pulse width modulation.
The cited U.S. Pat. No. 5,680,180 suggests a system with adjustable color discs for the adjustment of the color balance which is highly complicated by comparison.
A substantially simpler control of the color balance, however, is possible according to the invention in that the maximum of the current strength is achieved the moment a given color is generated, i.e. when a given segment of a color disc is present in the radiation path of the light rays used for the projection. It may be desirable, for example for the display of video sequences by means of a projection system with a color disc with segments in the three basic colors red, green, and blue as well as possibly white, i.e. white light transmitting segments, to put a stronger emphasis on the red component in the color mixing. For this purpose, the system is then controlled such that the lamp shines particularly brightly whenever the or a red segment of the color disc is in the radiation path. The adjustment of the color balance may accordingly take place exclusively through a suitable control and synchronization of the lamp current and the color disc rotation, according to the invention, which is advantageous because of the simplicity of the solution compared with other known solutions, for example solutions with different color discs for different application purposes (data display, for example display of tables, vs. video display with colors as natural as possible).
Whereas the realization of an exact control is comparatively complicated, the entire arrangement can be substantially more simply controlled if the maximum in the generation occurs exactly at a color, i.e. exactly in a segment.
A particularly advantageous embodiment of the method of generating a color sequence which comprises at least white is obtained when the control of the lamp current takes place such that the maximum current strength is reached when the color white is being generated. This has the major advantage, for example, that the fluctuation in the lamp brightness necessary for stabilizing the lamp arc does not lead to a change in color, but merely in the saturation of the color, the latter being hardly perceivable to the human eye.
A second advantage of this method of control is that the calculation of the grey tones (brightness values) in the generation of a digital picture can still be basedxe2x80x94as usual until nowxe2x80x94on a luminous flux which is constant in time, so that the calculation algorithms which have proven to be correct can be utilized.
A third advantage is finally that the fact that the highest brightness of the lamp falls in the region of projection of white light means that the time duration for generating the white component of the color sequence can be shorter, so that more time is available for the generation of the colored components of a pixel.
Preferably, the pulse duration corresponds substantially to the passage of the relevant segment. It is safeguarded thereby that exclusively this segment is influenced by the pulse, while the other segments are influenced by the non-pulsed range. It was found in this respect that a variation of xc2x15%, in particular of xc2x11%, at the start and end of the white generation in relation to the total duration of the white generation leads to satisfactory results.
It is advantageous for lamp stability as well as for a simple process control when the luminous flux supplied by the lamp disappears for a short period only or not at all.
The synchronization means serve to safeguard a color stability during projection. It is obvious that for such synchronization means all possibilities may be utilized which appear conceivable and practicable with respect to the means for generating the color sequences and the means for controlling the lamp operating current strength. For example, these means may be clocked by a common clock generator. Alternatively, one of these means, for example the rotating color disc, may provide a suitable clock, which is then correspondingly utilized.
A color adaptation may be carried out in a constructionally very simple manner through a suitable phase shift which may be enabled by the synchronization means. This is advantageous in particular for color balances which may be changed in time and which can be fine-tuned by means of a new calculation or supplementary calculation.
Preferably, the proportion of the time period during which the color white is generated during operation of the projection system lies between approximately 3 and 15%, preferably between approximately 6 and 12%, of the total duration of a color generating sequence. In this manner, comparatively much time is available for the generation of the actual colors. When a color disc is used, this means that the white segments may account for approximately 6 to 12% of the total surface area of the color disc and are accordingly penetrated by light from a HID lamp during approximately 6 to 12% of the total duration of the color disc revolution.